The present invention relates in general to electric cables, and in particular, although not exclusively, to electric cables for audio, hi-fi, video or computer applications.
It is a well-known technique to twist a pair of wires carrying an electrical signal to improve the noise rejection of the pair.
For computer applications, such as connecting peripherals to interface card, cables are known which comprise adjacent sets of twisted pairs, each pair consisting of one signal line and one ground wire. One configuration of such a cable is a ribbon-like flat cable in which there are flat untwisted regions at regular intervals along the cable for easy connection to crip-on connectors of the type used for ordinary ribbon cable. Because of the strobed data transfer protocol used on computer buses and in connections to peripherals, it generally is not necessary to use twisted pairs for all signal lines, instead just for the synchronising pulses and other strobing or enabling lines.
In audio systems applications it is a well-known technique to twist the pairs of conductors carrying differential signals in interconnecting leads (between for example the CD player and amplifier) and in speaker cables to improve the noise rejection of the cables. Spurious RF signals which would degrade the sound quality are rejected by the twisted geometry. In the case of speaker cables, it becomes particularly desirable to twist pairs of wires carrying the signals for each channel when the total cable length is great.
A variety of known audio cable geometries are shown in FIG. 1. In FIG. 1(a), two insulated wires 1, 2 with conductive cores 11, 21 are twisted together and encased in a flexible dielectric jacket 6. FIG. 1(b) shows a known geometry comprising two wires twisted together with a non-conductive strand 3, and FIG. 1(c) shows the cross section of a known geometry comprising two wires 1,2 twisted together with two non-conductive strands 3, 3a. 
In general, the greater the twist (i.e. the greater the number of twists per unit length) the greater the noise rejection property of the cable. However, as the number of twists per unit length is increased, there comes a point when the cable has a strong tendency to bunch even under tension, and once released from the spool on which it has been wound it becomes unmanageable. This has been referred to as the xe2x80x9celastic bandxe2x80x9d effect.
Partly for this reason, in the audio cable industry a twist frequency of 5 per inch has been seen as the benchmark for cables comprising twisted jacketed (i.e. insulated) conductors. It is a compromise figure, giving good noise rejection in conjunction with ease of manufacture.
It is an object of the present invention to provide an electrical cable which overcomes some of the problems associated with the prior art.
It is a further object of the present invention to provide an improved method of manufacturing an electrical cable.
According to a first aspect of the present invention there is provided an electrical cable comprising a first strand, a second strand, and a third strand, wherein said first and second strands are electrically conductive and electrically insulated from each other, said third strand is electrically non-conductive, and said first, second, and third strands are braided together.
Advantages of a cable in accordance with this first aspect of the present invention are numerous, and include:
1. The inductance per unit length of the cable is lower than that of a cable comprising the same conductive strands but in a twisted pair configuration (with the same crossover frequency). The xe2x80x9cbraidedxe2x80x9d geometry reduces the self inductances of the individual conductive strands.
2. The noise rejection of the cable is significantly better than that of an equivalent twisted pair. In particular the xe2x80x9cbraidedxe2x80x9d cable, whilst performing similarly to the twisted pair at rejecting noise caused by fluctuations in the component of background magnetic field transverse to the cable, is intrinsically better at rejecting noise caused by fluctuations in the longitudinal component.
3. The xe2x80x9cbraidedxe2x80x9d geometry can lead to significant reductions in the attenuation of signals along the cable compared with the twisted pair.
4. Braided cables in accordance with the present invention may exhibit lower resistance than equivalent cables comprising the same conductive strands but in the form of twisted or parallel pairs. This reduced resistance is due to reduced interaction between the adjacent xe2x80x9cgoxe2x80x9d and xe2x80x9creturnxe2x80x9d currents in a pair of conductive strands used to carry a differential signal, or, in other applications ac or dc power.
The xe2x80x9cgoxe2x80x9d and xe2x80x9creturnxe2x80x9d currents are moving in opposite directions and electromagnetic interaction between them distorts the current distributions in each conductive strand. This reduces the effective cross sectional area of the conductive part of the strand and so results in an increase in resistance. It should be noted that this is a real increase in the resistance of the cable, separate from any increase in the magnitude of the impedance of the cable due to any increase in its self inductance which may also result from changes in current distributions.
In a twisted pair or a closely spaced parallel pair the conductive strands carrying the xe2x80x9cgoxe2x80x9d and xe2x80x9creturnxe2x80x9d currents are in close proximity to each other along their entire lengths. The interaction between these currents is therefore strong, and results in increased resistance.
In contrast, in the inventive braided cables, the is conductive strands are repeatedly separated along the length of the cable by the non-conductive strand (or strands). This reduces the interaction between the go and return currents and so reduces any resultant resistance increase.
The thickness of the non-conductive strand or strands may result in a repeated spacing of the conductive strands between their xe2x80x9ccrossoverxe2x80x9d points that is sufficiently large to make resistance increases due to inter-strand interaction negligible, or even zero.
In a parallel or twisted pair arrangement, the separation of the conductive strands can be increased to reduce inter-strand interaction, but results in increased inductance and susceptibility to noise.
The inventive braided cables reduce inter-strand interaction, resulting in reduced cable resistance, whilst retaining low inductance geometry and improved noise rejection.
Interaction between adjacent xe2x80x9cgoxe2x80x9d and xe2x80x9creturnxe2x80x9d currents in cables has also been termed xe2x80x9cthe proximity effectxe2x80x9d, and for ac signals has been seen to result in increases in cable resistance with frequency. In contrast to twisted or parallel pairs, the inventive braided cables may render such resistance increases negligible, or even zero, for signal frequencies of interest (e.g. up to 20 KHz for audio applications).
In general, the proximity effect is more significant for low-resistance cables (i.e. incorporating heavier guage conductors) such as speaker cables, as lateral current mobility, which enables distortion of the current profile in the conductor, is greater. With larger diameter conductors, there is greater scope for current distribution distortion. Thus, in audio applications, the resistance reducing aspect of the inventive braided cables is particularly advantageous in speaker cables.
The strands may be encased in a flexible jacket of dielectric material. This jacket may comprise one or more materials from a list including PTFE, PE, and PVC, and may be a composite. Of course, a wide variety of other materials may be used.
As a result of the xe2x80x9cbraidedxe2x80x9d geometry lowering the self inductance of the cable, increased capacitance can be tolerated. Thus the cable designer has a wider choice of materials to use for the jacket, and so has more freedom to tailor the LCR properties of the cable to the particular application. For audio cables, the designer thus has more freedom to alter the xe2x80x9csoundxe2x80x9d of the cable.
Also, because higher cable capacitances can be tolerated, less expensive materials can be used for the jacket.
For certain applications, the jacket may be substantially non-flexible. For example, it may be desirable to prevent movement of the strands to reduce noise to a minimum.
The xe2x80x9cbraidedxe2x80x9d geometry also enables excellent levels of noise rejection to be achieved without the use of screening foils or braids, and so simplifies cable manufacture and reduces costs.
Of course, if screening means are incorporated, the noise rejection of the cable car, be improved still further.
The cable comprising three braided strands is more flexible than a cable comprising similar strands twisted together.
Also, a greater number of transpositions, or crossovers, of the two conductive strands can be achieved per unit length compared with a twisted geometry, without the cable becoming unmanageable, i.e., the xe2x80x9celastic bandxe2x80x9d problem is largely alleviated. Getting around the elastic band problem greatly improves the ease of manufacture of the cable, and by enabling the crossover frequency per unit length of the conductive strands used to carry electrical signals to be increased, the braided geometry can lead to improved noise rejection.
Even with the same crossover frequency per unit length, the braided cable of two conductive strands and a non-conductive strand can exhibit improved noise rejection over a cable comprising the same strands twisted together.
For a given crossover frequency, in the braided geometry the two conductive strands cross each other at a larger angle (i.e. closer to 90xc2x0)than do the conductive strands in a twisted pair configuration. This may contribute to the improved noise rejection of the cable.
Advantageously, the crossover frequency may be in the range 1 to 100 per meter. Of course, alternative crossover frequencies may be employed, and in cables comprising a plurality of groups of braided strands, the strands of one group may be braided with a different crossover frequency to those in another group.
By braiding the strands rather than twisting them, the inductance per unit length of the cable can be reduced, as can the attenuation of signals transmitted along the conductive strands.
By enabling reductions to be made in both the self inductance of the cable and the attenuation of signals, the braided geometry also enables the cable designer to have a greater level of control over the LCR properties of the finished design. For example, different insulating materials may be used which would, in a twisted geometry, lead to unacceptably high values of cable capacitance. However, by reducing L, increases in capacitance may be tolerated. In the case of audio cables, the design therefore has a greater degree of control over the xe2x80x9csoundxe2x80x9d of the cable, in particular a greater degree of control over the signal phasing. For cables predominantly carrying power, the designer has a greater degree of flexibility over the current flow. In addition, by giving the cable designer a wider choice of dielectric materials, the braided geometry enables cheaper materials to be used to lower the cost of the cable whilst retaining satisfactory performance.
By braiding the strands rather than twisting them the shape of the surface bounded by the two conductive strands is greatly altered. The fundamental difference between the shapes of these surfaces corresponding to the two geometries leads to the improved noise rejection of the inventive cable.
The braided geometry is well suited to flat ribbon-like cables comprising a plurality of strands. Advantageously such cables may be used in computer applications.
A cable comprising three braided conductive strands could of course have the same geometry as an embodiment the present invention, and indeed such cables are known, and used for example as interconnects in hi-fi applications. One of the conductive strands could be left unconnected, while the other two were used to carry signals. This arrangement might be expected to show similar characteristics to the cable in accordance with an embodiment of the present invention. However, in cables according to the present invention, using a non-conductive third strand enables considerable savings in cost to be made. This is true for many embodiments of the present invention but is particularly important when expensive, high conductivity materials are used in the conductive strands, especially when these conductive strands are heavy gauge wires. In addition, use of a non-conductive third strand enables the weight of the cable per unit length to be reduced. Furthermore, by incorporating a third strand comprised entirely of dielectric material, rather than using, say, a conductive strand in the form of a wire with an outer sheath of dielectric around a conductive (but unused ,i.e., unconnected) core, an increased amount of dielectric material can be incorporated in the cables cross section. This is yet another factor which gives the designer greater control over the electrical properties of the cable.
Advantageously, the third strand may comprise PTFE, PE, or PVC, but it will be apparent that a wide variety of alternative materials may be used. The third strand may be a composite.
The third strand and the flexible jacket may comprise the same dielectric material, and may be substantially integral, i.e. the boundary between the third strand and the jacket may be indiscernible for at least part of its length.
Advantageously, insulation of the first and second conductive strands may be achieved by using a first strand comprising a conductive core with an outer sheath or coating of dielectric material. In this case, the second strand may not require separate insulation, and could be a bare strand of conductive material.
Alternatively the second conductive strand may also have a dielectric sheath.
It will be apparent that the first, second and third strands may, independently, have many different configurations, and all combinations are possible.
For example, the conductive strands may comprise a single conductive filament or a plurality of conductive filaments, formed from a variety of materials including copper, oxygen-free copper (OFC), silver, pure silver, gold and conductive carbon fibre. Of course this list is not exhaustive, and a wide variety of other materials may be employed. Advantageously, the filaments may be coated or plated, for example with silver or gold.
The conductive strands may comprise a conductive core inside a dielectric sheath, and this sheath may comprise polytetrafluoroethylene (PTFE), polyethylene (PE), polyvinylchloride (PVC) or any other suitable material. The sheath may be a composite.
The conductive strands may be round wires, or alternatively tapes or wires with other cross sections.
The non-conductive strand may be comprised of a single dielectric material or may be a composite of at least two dielectrics. Suitable dielectrics include PTFE, PE and PVC which may be chosen to give desired cable characteristics. A material may be chosen to given a cable with increased capacitance per unit length. Again, the third strand may be round, or alternatively have a different cross section.
By xe2x80x9cnon-conductivexe2x80x9d it is meant that the third strand is incapable of carrying an electrical current from one end to the other. It may be comprised entirely of dielectric material, or alternatively may have regions of conductive material embedded in it, electrically insulated from each other, but capable of affecting the LCR characteristics of the cable.
The cable may be a cable of twenty-five strands, and may be in the form of a ribbon cable for computer applications.
All three strands may be round and have the same diameter, giving a symmetrical uniform cable, or alternatively at least two of the strands may have different diameters. Thus it will be apparent that strands may independently have a variety of cross sections and sizes.
Advantageously the cross sectional areas of the conductive components of the first and second conductive strands may be the same, or the second strand may have a reduced conductive cross section. This may act as a choke in the xe2x80x9creturnxe2x80x9d wire of a speaker cable to give desired sound quality.
Advantageously, the three strands may be encased in a dielectric jacket.
Advantageously, a cable may comprise two sets of strands, each set comprising three strands braided together where one of the strands in each set is non conductive. The two sets of strands may be twisted together, with or without filler material, and encased in an outer sheath.
Shielding means may be incorporated in the cables to improve noise rejection, and may, for example, be in the form of a foil or an outer braid of conductive filaments.
Advantageously, cables in accordance with embodiments of the present invention may comprise an inner section (i.e. xe2x80x9ccorexe2x80x9d) of conductive and non-conductive strands braided together, surrounded by one or more braided sleeves. The braided sleeves may comprise two or more electrically conductive strands and may be used to carry signals different from those carried by the inner section.
A surrounding braided sleeve may comprise conductive and non-conductive strands braided together, or just conductive strands, or just non-conductive strands.
Cables in accordance with aspects of the present invention may be used to connect various electrical devices, for example in audio, hi-fi, video and computer systems, and combinations thereof.
According to a second aspect of the present invention there is provided a method of manufacturing an electrical cable comprising the steps of braiding together a first conductive strand, a second conductive strand and a non-conductive strand, and electrically insulating said first and second strands from each other.
Embodiments of the present invention will now be